1. Field of the Invention
The present invention relates to a cordless telephone, and more particularly to a cordless telephone comprised of a base unit including a power source circuit and a handset unit including a secondary battery which is charged by means of power supplied by the power source circuit and a power failure detection circuit for detecting a break in power at the power source circuit to thereby enable the handset unit to be changed to sleep mode, thus conserving power at the handset unit.
2. Description of Prior Arts
As shown in FIG. 1, a conventional cordless telephone is comprised of a base unit 1 and a handset unit 2 wherein a secondary battery 21 contained in the handset unit 2 is charged by means of a power source circuit 11 contained in the base unit 1 with electric power being supplied to a cordless telephone circuit 22 of the handset unit 2 from the secondary battery 21. It is to be noted that the numeral 12 designates a cordless telephone circuit of the base unit 1 which transmits and receives signals to and from the cordless telephone circuit 22 of the handset unit 2 in a wireless manner.
In the above-mentioned conventional cordless telephone, if a power failure occurs at the power source circuit 11 of the base unit, power residing in the secondary battery 21 is consumed at the handset unit when the cordless telephone circuit 22 is in an intermittent receipt operation, despite the fact that the secondary battery 21 does not receive power from the power source circuit 11 of the base unit 1. Accordingly, if break in power lasts for a long period of time, the charge in the secondary battery 21 is depleted, with the result that the telephone cannot be used. This may causes serious inconvenience particularly in an office or the like. That is, if a main power switch in an office is turned-off upon office workers leaving the office and the main power switch is turned-on the next morning for instance, the telephone cannot be used until the battery has recharged. That is, the telephone cannot be used immediately after the turn-on of the main power switch, accordingly, during this unserviceable period of time, some business may be interrupted and put to an inconvenience.
Accordingly, it is desirable to reduce power consumption in a secondary battery provided in a handset unit when power supply to a base unit is interrupted.
Conventionally, the following method has been employed to detect a break in power in a power source circuit in a base unit with a view to reducing power consumption of a secondary battery in a handset unit.
(1) A mechanical switch is provided which detects whether or not the handset unit is placed in a cradle of the base unit, and a voltage detection circuit is provided which detects whether or not a voltage at a charging terminal of issued the handset unit for charging a secondary battery of the handset unit from a power source circuit of the base unit is lower than a predetermined level. When the mechanical switch detects that the handset unit is placed in the cradle and the voltage detection circuit detects that the voltage at the charging terminal is below the predetermined level, a power supply switch connecting the secondary battery to a reception circuit of the handset unit will be turned-off to cut power supply to the reception circuit. Such a method and apparatus as mentioned above are disclosed in Japanese Patent Laid-Open Publication No. 3-212129 issued on 1991.
(2) A voltage detection circuit is provided which detects a charge in a secondary battery of a handset unit and a voltage divider circuit is provided which divides the output voltage of the voltage detection circuit and outputs the divided voltage only when a break in power at a power source circuit of a base unit occurs under the condition that the handset unit is placed on a cradle of the base unit. When a break in power at the base unit is detected on the basis of the divided voltage, a switch inserted between the secondary battery and a load circuit will be turned off so as to shut down supply of power to the load circuit. Such a method and apparatus as mentioned above are disclosed in Japanese Patent Laid-Open Publication No. 3-71758 issued in 1991.
According to the prior example (1), it is necessary to provide a mechanical switch for detecting if a handset unit is placed in the cradle. Accordingly, since it is necessary to provide not only two pair of charging terminals for charging the secondary battery from the base unit but also the mechanical switch, it makes a constitution of a cordless telephone complex.
According to the prior example (2), when a break in power at a power source circuit of a base unit is detected, power will not be supplied to the entire load circuit in a handset unit, and as a result information previously stored in a memory of the handset unit will be lost.